One object in developing implantable defibrillation apparatus has been to lower the shock strength produced by that apparatus so that the size of the shock capacitor, and hence the size of the implantable apparatus itself, can be reduced. Several approaches to achieving this goal have been taken. U.S. Pat. No. 4,780,145 to Tacker et al. discusses the problem with single-pulse defibrillation systems in that the current density between the electrodes is not uniform throughout the ventricles. Tacker describes a sequential-pulse, multiple current pathway defibrillation method in which two defibrillation pulses are delivered along different current pathways.
U.S. Pat. No. 5,536,764 to Adams et al. and U.S. Pat. No. 5,344,430 to Berg et al. both describe implantable defibrillation systems employing two or more successive pulses, but again all pulses are defibrillation pulses. Similarly, U.S. Pat. No. 5,324,309 to Kallok describes successive defibrillation pulses that overlap in time. Adams et al. point out that, after four separate defibrillation attempts, therapy is terminated because conversion thresholds increase with time in a fibrillation episode, and that patients are likely to suffer brain damage after prolonged fibrillation. Hence, it is extremely desirable to increase the likelihood of successful defibrillation on an early attempt: a goal not always consonant with that of decreasing shock strength.
Other implantable defibrillators employ pacing, or pretreatment, pulses. U.S. Pat. No. 5,366,485 to Kroll et al. and U.S. Pat. No. 4,559,946 to Mower et al. both describe defibrillation apparatus in which pacing or pretreatment pulses are delivered through the same electrodes as the defibrillation pulse. U.S. Pat. No. 4,693,253 to Adams and U.S. Pat. No. 5,431,682 to Hedberg both describe defibrillation apparatus in which pacing pulses are delivered after defibrillation. U.S. Pat. No. 5,282,836 to Kreyenhagen et al. describes an atrial defibrillator wherein pacing pulses are delivered through a pacing electrode prior to defibrillation pulses being delivered through a separate set of defibrillationelectrodes.
U.S. Pat. No. 5,489,293 to Pless et al. describes an apparatus for treating cardiac tachyarrhythmia which uses a lower voltage defibrillation apparatus by providing a rapid sequence of defibrillation shocks synchronized with sensed sequential cardiac or electrogram events or features during an arrhythmia.
U.S. Pat. No. 5,464,429 to Hedberg et al. describes an apparatus in which a stimulation pulse is delivered through an electrode that ordinarily serves as a pacing electrode, with the stimulation pulse being delivered prior to a defibrillation pulse (the latter being delivered through separate defibrillation electrodes). The stimulation pulse is of a magnitude greater than that of a pacing pulse, but less than that of a defibrillation pulse, and is said to produce a refractory area around the stimulation electrode. However, the stimulation pulse is delivered via an electrode that also serves as a pacing electrode, rather than an electrode specifically positioned in a weak field area of the defibrillation electrodes. The use of a stimulation pulse of a reverse polarity to the first phase of a biphasic defibrillationpulse is not disclosed.
U.S. Pat. No. 5,282,837 to Adams et al. (InControl, Inc.)(see also Divisional application Ser. No. 5,282,837) describes, in FIG. 1 and accompanying text, an atrial defibrillator and method in which a lead 36 is inserted into the coronary sinus so that a first tip electrode 42 is within the coronary sinus adjacent the left ventricle, a second ring electrode 44 is within the coronary sinus beneath the left atrium, and the third electrode 46 within the right atrium or superior vena cava. The first electrode serves as a sensing electrode, the second electrode (still in the coronary sinus) serves as both a sensing and defibrillating electrode, and the third electrode serves as a sensing and defibrillating electrode (see Col. 5 line 57 to Col. 6 line 12).
U.S. Pat. No. 5,433,729 to Adams et al. (corresponds to PCT WO92/18198) is a CIP of Adams '837. Adams '729 describes, in FIG. 9 and accompanying text, a lead system 250 configured in accordance with that described above. A first (right ventricle) lead 252 includes an elongate large surface area electrode 256, a distal or tip sense electrode 258, and a ring or proximal sense electrode 260. Sense electrodes 258, 260 are positioned in and in contact with the wall of the right ventricle, and elongate electrode 256 is in the right atrium. A second (coronary sinus) lead 254 includes a tip, or distal sense electrode 264, a ring or proximal sense electrode 266, and a second elongate, large surface area electrode 262. Distal and proximal sense electrodes 264, 266 are both adjacent the left ventricle within the great vein, and elongate electrode 262 is within the coronary sinus beneath the left atrium. The right ventricle sense electrodes 258, 260 are coupled to inputs 50a, 50b of first sense amplifier 50; the great vein sense electrodes 264, 266 are coupled to inputs 52a, 52b of second sense amplifer 52. This is to provide sensing of the right ventricle and the left ventricle, and the non-coincident sensing of the depolarization activation waves. for synchronizing delivery of energy to the atria (see column 15 line 34 to column 16 line 54; column 5 lines 62-64).
U.S. Pat. No. 5,014,696 to Mehra (Medtronic Inc.) describes an endocardial defibrillation electrode system in which a coronary sinus electrode extending from an area adjacent the opening of the coronary sinus and terminating in the great vein is used in combination with subcutaneous plate electrodes and with right ventricular electrodes. The coronary sinus electrode 78 encircles the left ventricle cavity 86 (Col. 5 lines 50-51; FIG. 5B). It is stated "it is important not to extend the electrode 78 downward through the great vein 80 toward the apex 79 of the heart" (col. 5 lines 28-30). U.S. Pat. No. 5,165,403 to Mehra (Medtronic, Inc.) describes an atrial defibrillation electrode 112 that is located "within the coronary sinus and the great cardiac vein."
U.S. Pat. No. 5,099,838 to Bardy (filed Dec. 15, 1988; Medtronic, Inc.) describes a defibrillation electrode in the great vein that is used in combination with subcutaneous plate electrodes and with right ventricular electrodes (col. 1 line 65 to col. 2 line 2). With respect to the great vein electrode, it is stated at column 5, lines 20-33 therein: "When so mounted, the elongate defibrillation electrode 78 extends from a point adjacent the opening of the coronary sinus 74 and into the great vein 80. This provides a large surface area defibrillation electrode which is generally well spaced from the ventricular defibrillation electrode 74 and provides good current distribution in the area of the left ventricle 77. It is desireable to extend the electrode 78 around the heart as far as possible. However, it is important not to extend the electrode 78 downward through the great vein 80 toward the apex 79 of the heart, as this will bring the coronary sinus and right ventricular electrodes into close proximity to one another, interfering with proper current distribution. U.S. Pat. No. 5,193,535 to Bardy (filed Aug. 27, 1991) also describes a great vein electrode. At column 7, lines 31-35, it is stated: "The coronary sinus lead is provided with an elongated electrode located in the coronary sinus and great vein region at 112, extending around the heart until approximately the point at which the great vein turns downward toward the apex of the heart."
U.S. Pat. No. 5,431,683 to Bowald et al. (Siemens) describes a ventricular defibrillation electrode system in which on electrode is placed through the coronary sinus into a peripheral vein of the heart. The term "peripheral vein" is defined therein as to encompass "the venous side of the coronary vessels running between the base and the apex of the heart. The [sic] include the middle and small cardiac veins, and the portion of the great cardiac vein which runs between the base and apex of the heart.
The definition of "peripheral veins" used herein, therefore, excludes that portion of the great cardiac vein which runs along the base plane of the heart, which has been used [as] a site for electrode placement in prior art electrode systems." The electrodes are in the shape of a helix to apply pressure against the inner wall (col. 4, lines 14-17), with blood being able to flow unobstructed through the interior of the helix (column 4, lines 46-48)(See also U.S. Pat. No. 5,423,865 to Bowald). Such stent-type electrodes can be difficult to adjust or remove. Only a simple shock pattern is described in Bowald, and efficacious electrode configurations and shock patterns are neither suggested nor disclosed.
U.S. Pat. No. 5,690,686 to Min et al. (Medtronic Inc.) describes an atrial defibrillation method in which a coronary sinus/great vein electrode is coupled to a right atrial/superior vena cava electrode and a subcutaneous electrode in the form of the housing of an implantable defibrillator. The device is stated to be preferably practiced as a combined atrial/ventricular defibrillator (col. 2, lines 26-35).
In view of the foregoing, a first object of the invention is to provide an implantable system for treating cardiac arrythmia that does not require invasion of the chest cavity for the placement of epicardial electrodes.
A second object of the invention is to provide an implantable cardioversion system wherein the probability of successful cardioversion on administration of the first cardioversionpulse is enhanced, particularly in the case of ventricularfibrillation.
A third object of the invention is to provide an implantable system for treating cardiac arrythmia that can enable reduction of cardioversion, and particularly defibrillation, shock strength.